Hydraulic servomotor



Patented Oct. 14, 1952 HYDRAULIC SERVOMOTOR Lawrence Farnell Mott, Walton, Stone, England, assignor to H. M. Hobson Limited, London, England, a company of Great Britain Application May 27, 1950, Serial No. 164,760 In Great Britain June 10, 1949 Claims. 1

Hydraulic servomotors are normally fitted with a control valve which is movable in opposite directions from a neutral position to effect alternative pressure and exhaust connections to opposite sides of the motor, thereby causing the motor to move an output member in a direction, and to an extent, determined by the movement imparted to an input member linked to the control valve.

Where a servomotor of this kind is employed for positioning the control surface of an aircraft, serious danger can arise in the event of a control valve seizing in an open position, and thereby causing the pressure to drive the control surface to an undesired extreme position.

U. S. application, Serial No. 164,654, now Patent No. 2,597,418, discloses mechanism for safeguarding against this contingency, embodying a collapsible member which, when the control valve seizes, collapses and thereby actuates mechanism for rendering the servomotor inoperative and permitting manua1 control of the output member.

This invention approaches the problem of valve seizure on different lines, and provides a hydraulic servomotor embodying duplicated control parts, namely a normally operative control valve and an emergency control valve, which is movable, as the result of effort exerted on th input member when the normally operative control valve seizes, to take over control of the output member and permit of servo operation thereof from the input member.

With the control valve provided in accordance with the invention, therefore, the servomotor is not rendered inoperative, as in the above-mentioned cases, but can continue to work as a servomotor under the control of the emergency control valve.

In one form of servomotor according to the invention the normally operative control valve slides in a ported sleeve which is biassed so that it its held stationary so long as the valve can move freely in it but is caused to move in relaation to its housing, in the event of the valve seizing in the sleeve, and, when so moved, first reverses the pressure and exhaust connections established to the servomotor by the seized valve and thereafter permits of control of the servomotor, through the ports in the sleeve, in accordance with the control exerted over the input member.

In an alternative arrangement, the emergency control valve is disposed parallel to the normally operative control valve and linked to it and to the input member, the emergency control valve being normally resiliently held against movement but being movable, if the other control valve seizes, first to reverse the pressure and exhaust connections to the servomotor established by the seized control valve and thereafter to take over control of the servomotor.

With the servomotor according to the invention irreversibility is maintained when the normally operative valve seizes and the emergency valve takes over from it. Also if the seized valve frees itself normal functioning is restored,

Two embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which- Fig. 1 is a diagram of the first form of servomotor, and

Fig. 2 is a diagram of the second form of servomotor.

Like reference characters designate like parts in the two figures.

The servomotor shown in Fig. 1 comprises a cylinder 3 movable in relation to a fixed piston 4 and carrying a valve housing 5. The piston rod 6 is attached, at I, to a fixed structure and the cylinder is coupled to an output member 8. In the housing- 5 is disposed a sleeve 9 within which is a slidable piston valve I0. The sleeve 9 is provided with centering springs H, one at each end, which normally hold the sleeve in a central position and prevent the sleeve from moving when the piston valve In is moved axially in the sleeve 9 by a manually operable input member 12. Each spring H bears against a washer [3 which, in turn, bears against a retaining cover l4 secured to the housing 5 by screws [5. I Secured to the ends of the sleeve 9 are collars l6 which bear against the washers 13.

Liquid under pressure is admitted to the housing through an inlet ll communicating with a central inlet port [8 and two additional inlet ports [9 to the bore containing the sleeve, the additional inlet ports l9 being equally spaced at opposite sides of the central port 18 and normally masked by the sleeve 9, as shown.v Liquid is returned to the source of supply from an outlet 20 which is connected to outlet ports 2| from the housing bore, these being outside, and spaced equally from, the respective additional inlet ports IS. The sleeve 9 has an inlet port 22 normally coincident with the central inlet port 18 in the housing and two outlet ports 23 normally coincident with the outlet ports 2| in the housing. Emergency ports 24 in the sleeve lie midway between each of the additional inlet ports I9 and the adjoining outlet port 2 I.

The ports I8, I9, 2I, 22 and 23 are all of the same width w. The emergency ports 24 in the sleeve are each of width 2w. The outer edge of each emergency port 24 in the sleeve is at a distance w from, the inner edge of the adjoining outlet port 23 in the sleeve, the inner edge of each outlet port 2| is at a distance 4w from the outer edge of the adjoining additional inlet port I9 and the inner edge of each additional inlet port I9 is at a distance at least 311; from the adjoining edge of the central inlet port I 8.

The piston valve I II has two waisted sections 25 formed upon it between lands. 26 has a width equal to or slightly greater than that of the inlet port 22 in the sleeve while the width of each waisted section 25 is equal to or slightly less than the distance between the adjacent edges of the associated inlet and outlet ports 22, 23 in the sleeve.

Onthe opposite side of thesleeve to that carrying the ports 22, 2-3, 24' are formed slots 21', each communicatingwith a port 28- formed in the housing. These ports 28- are of a width w and each is located midway between the inlet. port 22 in the sleeve and one of the outlet ports 23 in. the sleeve. The slots 21 are of width 511: and also are located midway between the inlet and outlet. ports. 22, 23 in the sleeve. The ports 28 connect. with passages 29' which conduct liquid to and from opposite ends of the servo cylinder 3.

The sleeve. 9 may be slid axially a distance of 2w in; either direction from the central position by'compression ofone of the springs, I I. Further movement is, prevented by one or other of. the washers I3 abutting against a surface 33 on the housing 5.

The piston valve I may be slid axially by an amount 220. in both directions relative to the housing, provided the sleeve 9 is ina central position, movement being limited by abutment. of collars 3I-v formed on the piston valve; against the retaining covers HI. If the sleeve 9 is displaced by some amount, in one direction, the piston valve I.0- may be displaced the full amount 2w. in the samedirection, but the collar I6 at the end of the sleeve!) which projects from thehousing will limit. displacement of the piston valve' I0 inv the other direction toan amount equal to the difference between 2w and the sleeve displacement.

In: normal operation the sleeve. 3 remains stationary,,anclcontrol of the servomotor is effected by'movement of the piston valve I0. Displacement of it in either direction causes thecentral land 26 to uncover the inlet: port 22 and admit liquid to one passage 29, while liquid. returning along the other passage 29 passes to the outlet 20 by virtue of one of the end lands 32' of' the valve uncovering the associated outlet port 23. Thus, if the valve In is moved to the left liquid isadmitted from the inlet port 22 to the left hand end of the cylinder 3 and the right hand end of the cylinder 3 is connected to the left hand. outlet port 23. Consequently the cylinder 3 moves to the left. As the cylinder moves, the housing and sleeve 9 travel with it in the same direction as that in which the piston valve ID was moved, the movement ceasing when the sleeve has caught up with the displaced piston valve, thereby closing the inlet port 22 and the open outlet port 23.

If the piston valve I0 seizes within the sleeve when displaced by any distance, say to the left as seen in Fig. 1 up to the maximumof 2w, the

The central land operator will resist undesired continued motion of the servomotor by applying force in the opposite direction to the displacement of the piston valve, which will cause the sleeve 9 with seized valve I0 therein to slide within the housing in the opposite direction to the displacement of the. piston valve relative to. the sleeve, i. e. to the right. When the sleeve has travelled a distance of w to the right, the inlet port 22 in the sleeve will no longer be open to the inlet port I8 in the housing thus cutting off the supply of liquid to the motor. Similarly the outlet ports 23 in the sleeve. will. no longer be aligned with the outlet ports, 2| in the housing, thus also cutting off the flow of liquid leaving the motor. Further displacement. of the sleeve 9 causes the left hand emergency port 24 to start to uncover the associated additional inlet port I9 in the housing, thus admitting pressure liquid to the left hand port 28 which was previously connected, by movement to the left of the pistonvalve II], with the outlet 20, while. the. other emergency port 24. simultaneously starts to uncover the right hand outlet port 2| in the housing, permitting liquid to be exhausted from the other port 28. which was previously connected to the inlet ll. The direction of motion of the servomotor is thus reversed.

The motion of the servomotor in the reverse direction will cease. when the housing 5, travelling to the right with the servomotor, overtakes the sleevefi'and closes the ports 24 therein. The operator will thus retain control over the servo motor, with a small change in the datum position of the output. member 8 in relation to the input. member I2 occasioned by the displacement of: the sleeve 9 in relation to the housing 5. If the piston. valve I 0 frees itself, the springs II will return. the sleeve 9' to central position and thepistonvalve It) will resume control.

Shouldv the piston valve I3 seizein the sleeve 9. at the mid-position. when there is no flow to or from the motor, the sleeve 9 must be moved a distance in either direction before the servomotor will commence to move inthe corresponding direction.

In the alternative form of the inventionshown in Fig. 2, the housing has formed within it two cylindrical bores in which slide two cylindrical piston valves I0, 40, the axes of. which are substantially parallel. The piston valves I0, 40 are pivoted attheir. ends to a link 33 which ispivoted at an intermediate point 34 to the input member I2. The piston valve 40 is constrained from sliding axially by means of springs I'I bearing against washers I3 which in turn abut. against covers I4 secured to the housing 3. Collars I6 formed on the piston valve 40 engage with the washers I3 and axial movement can occur' only when the force applied to the piston valve 40' is sufficient tov overcome the springs II.

Liquid under pressure is admitted at the inlet IT andconducted as before to an inlet port I8 and additional inlet ports I9- in-the housing. Exhausted liquid returns to the source of supply from outlet which is connected to outlet ports 2!. and. additional. outlet ports in the housing. Complementary to ports I8, I9, 35 and 2I are ports. 36, 31,, 3B. and 39 respectively, communication between them. being, possible only when grooves 4t, 42 and 43 in the valve 46' register with the ports.

The ports and grooves are disposed in. a particular manner with regard to each. other. In the. mid-position ports I8 and 36 are aligned with groove 41 and each pair of ports 2| and 39 is aligned with a groove 43.

All the ports and grooves have the same width w. Adjacent edges of each pair of ports 2| and 35' are about 2w apart, and adjacent edges of each pair of ports [9 and I8 are about 3w apart, while'the adjacent edges of each pair of ports 19 and 35 are also 3w apart. Each groove 42 is located midway between one pair of ports l9 and;

The port 36 communicates with another port 44 opening into the bore of the piston valve 10 and normally masked by the central land 26 thereof. The ports 39 connect with corresponding ports 45 normally masked by the end lands 32ofrthe valve l0, while each pair of ports 31 and 38 connects via a common passage with a port 46 communicating with one of the waisted portions of the valve 10.

In normal operation, the piston valve 40 is retained stationary by its springs H and movement ofthe input member l2 serves only to displace the piston valve l0. Theland 26 then uncovers the port 44 and admits liquid under pressure to one of the passages 29 connected to the servo'cylinder. Simultaneously one of the lands 32 uncoversits outlet port 45 and connects it to the other passage 29 to exhaust liquid from the servomotor to the outlet 20. Should the piston valve l0 seize within the housing 5 when displaced from the mid-position, e. g. to the left,

the operator will resist undesired motion by application of force to the input member 12 causing a force to be applied to the piston valve 40 which, if sufiicient to overcome the appropriate spring II, will displace the piston valve 40 in the opposite direction, 1. e. to the right. When piston valve 40 has moved a distance 20 to the right,.the pressure supply to the port 44 is cut off and simultaneously exhaust liquid from the left hand port 45is prevented from escaping, thus causing undesired motion to cease. Further movement to the right of the piston valve 40 will place the right hand ports 35 and 38 in communication via the right hand groove 42 and simultaneously the left hand ports l9 and 31 will be connected by the other groove 42, so that the flow of liquid to the servo cylinder is reversed in direction.

If the piston valve I0 seizes within the housing in the mid-position, with the land 26 covering the port 44, the piston valve 40 must be moved a distance w in each direction before the servomotor commences to move in the corresponding direction.

The piston valve 40 has a maximum permissible movement of 2w in each direction from the mid-position, while the piston valve 10 has a movement of w in either direction from the midposition. Movement of the input member 12 relative to the housing 5 is limited by stops 50, coacting with a cover plate 5|, to an amount which is so proportioned in relation to the point of connection of the input member [2 to the link 33 that the piston valve l0 may just travel the full amount w permitted by collars 3| on it. Application of force to the input member 12 can only move the piston valve 40 when the piston valve [0 is not free to slide within the limits permitted by its collars 3|.

While in the examples which have just beenf described, the control valve serves to control pressure and exhaust connections to opposite ends of a servo cylinder, it will be appreciated that the control valve may, as an alternative, control pressure and exhaust connections to a hydraulic motor, for example as described in the above mentioned Patent No. 2,597,418- so as to cause the hydraulic motor to position the output member in accordance with the position assumed by the input member.

What I claim as my invention and desire to secure by Letters Patentis:

1. In a hydraulic servomotor, a, control valve assembly comprising a valve housing, a sliding piston valve, an input member coupled to'the piston valve, a ported sleeve surrounding the valve and normally held by springs in a central position, in relation to the housing, pressure and exhaust connections to the housing, and a pair of outlets from the housing to the servomotor, the sleeve having a pair of normally operative ports through which, on sliding movement of the piston valve in the sleeve in either direction from a neutral position, alternative pressure and exhaust connections are established to the outlets according to the direction of movement'of the piston valve and also a pair of emergency and normally inoperative ports, said sleeve being movable against its springs, in the event of valve seizure and by effort applied to the input member, in the direction opposite 'to that in which the'piston valve has been displaced, to position to render said normally operative ports ineffective and to render said emergency ports effective to establish, in accordance with the direction of movement of the sleeve, alternative pressure and exhaust connections to the outlets.

2. In a hydraulic servomotor, a control valve assembly comprising a valve housing, aninput member, a pair of parallel sliding piston valves mounted in said housing, a linkage connecting said valves together and to the input member, springs normally holding one of said valves against movement in relation to the housing, pressure and exh'aust'connections to the housing, passages in the housing, and a pair of outlets from the housing, said spring loaded valve having grooves through which, on sliding movement of the other valve in either direction from a neutral position, alternative pressure and exhaust connections are established through certain of said passages to the outlets according to the direction of movement of said valve and said spring loaded valve being movable against its springs, in the event of seizure of said other valve in said housing and by effort applied to the input member, in a direction opposite to the direction of displacement of the seized valve, to position to render said grooves ineffective and to render a pair of normally inoperative emergency grooves in the spring loaded valve effective to establish through other of said passages, in accordance. with the direction of movement of said spring loaded valve, alternative pressure and exhaust connections to the outlets.

3. A hydraulic servomotor, comprising a servo member, a housing, a cavity in said housing accommodating said servo member, a pressure inlet to said housing, an exhaust outlet from said housing, an input member, a main control valve mounted to slide in said housing and connected to said input member, said main control valve normally constituting the sole controlling agency for the servomotor and being movable by said input member in opposite directions from a neutral position, passages in said housing controlled by said main control valve, said passages being closed when said main control valve is in its neutral position but effecting, on movement of said main control valve from its neutral posi- 7 tion,- alternative connectionsbetween the inlet and outlet". and the opposite ends of said caw'ty according to, the directionof movement of said control valve, anemergency control valvev operatively connected .tofsaid input member, said emergency control valve being; movable in said housing in opposite directionsfrom a neutral position, further passages in said housing controlled: by said emergency control valve, said further passages being closed. when said emergency control valve is in its neutral position but effecting, on movement of. said emergency controlvalve from its neutral position, alternative connections between the inlet and outlet and opposite ends of said cavity, and: means restraining saidemergency control valve against movement. from said neutral position, said restraining'means yielding, in the event of seizure of the maincontrol valve against movement relatively to said. housing, to permit of displacement of said emergency control valve by said input member.

4; A hydraulic servomotor, comprising a servo member, a housing, a cavity in said housing accommodating said servo member, a pressure inlet to said housing, an. exhaust outlet from said housing, an input member, a sleeve mounted to slide: in saidhousing, biasing means normally restraining said sleeve against movement from a central position in said housing, a control valve coupled to the input member and mounted to slide in said sleeve, passagesin said housing, and ports in said sleeve, said passages and ports serving, under control of said control valve, to establish alternative connections between the pressure. inlet and the exhaustv outlet and opposite. ends of said cavity according to the direction. of movement of said control valve from a neutral, position, andsaid sleeve. being movable by said. input member, on seizure of said main control valve in said sleeve, to establish through said portsand'passages alternative pressure and exhaust connections. to. said cavity and thereby to effect operation of theservomotor under control of. said input member.

5. A hydraulic servomotor, comprisinga: servo member, a housing, a cavity insaid housing accommodating said servo member, a pressure inlet to said housing, an exhaust outlet fromsaid housing, an input member, a main control valve mounted to slide in said housing, connected to said input member and movable thereby in opposite directions from a neutral position,.'passages in said housing, said main control valve being effective when moved by said input memher to establish through certain. ofv said. passages alternative conections; between. the pressure inlet and the exhaust outlet and opposite ends of said cavity according to the direction of'movement of said main control valve from its neutral position, a normally inoperative emergency control valve mountedto slide in said. housing, an operative connection. between said emergency control valve and said input member, and. resilient meansfor normally preventing movement of said emergency control valve by said. input member, said resilient means yielding, in the event of seizure of said main control valvein said housing, to permit of movement of said emergency control valve by said input member and thereafter to establish through others' of said passages alternative pressure. and exhaust connections to said cavity permitting of operation 01. the servomotor by said input member;

LAWRENCE FA'RNELL MOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED. STATES PATENTS 

